At least 20 genetic loci contribute to disease initiation and progression in the NOD mouse model for spontaneous autoimmune diabetes. One way to dissect this genetic complexity is to isolate an important step (trait) in the pathogenesis of the disease and study both mechanistic and genetic features of this trait. We have chosen a specific trait: sensitivity to CD8 + T cells that react to a known pancreatic antigen, insulin, in the context of MHC class I molecule K d. The cells we use are pre-activated in vitro and cause diabetes in NOD mice within a week upon injection. Thus, we circumvent the requirements for activation in vivo, and can address the question of what makes a mouse sensitive or resistant to the diabetogenic action of the insulin-specific CD8+T cells (IS-CD8 + cells) in a robust experimental setting. Preliminary experiments have shown that chemoattraction and recognition of the cognate peptide expressed by endothelial cells are the key mechanisms in the homing of IS-CD8 + cells. In addition, some Kd-expressing strains of mice were found to be resistant to IS-CD8 + cells. Further analysis has suggested that at least two different mechanisms are involved. The resistance is based either on the inability of T cells to penetrate the islets or on the loss of ability to generate cognate peptide. Investigation of the mechanism of the accessibility of islets to T cells and its genetic control is the goal of the current application. Specific Aim 1. Study the mechanism of homing of insulin-specific T cells to the islets. Our primary effort will be invested in studies of the interactions between the activated insulin-specific T cells and the endothelial cells in the pancreatic capillaries. We will study the mechanisms involved in presentation of pancreatic antigen by endothelial cells. Specific Aim 2. Study the mechanism of resistance to anti-insulin cytotoxic T lymphocytes revealed through genetic screens. We will focus on a gene that is linked to MHC in B10.D2 mice and study the mechanism of the loss of cognate peptide presentation in H-2k-positive mice. Identification of the genes involved will provide us with crucial information on what molecular pathways are engaged and will allow further elucidation of the homing of autoimmune T cells.